Rotary retort furnaces have long been used for the continuous heat treatment of a variety of small parts, such as screws, nuts, bolts, studs, nails and washers. These furnaces, even those without an internal auger flight, are particularly well suited for the processing of such small parts as, in addition to providing continuous operation, the rotary conveying action tends to tumble the parts breaking up any jams or tangled clumps of parts thereby facilitating better and more thorough heat treatment of each of the individual parts. Unfortunately, the cost of manufacturing and maintaining the prior art rotary retort furnaces is not always commensurate with the economy of heat treating such small parts. Furthermore, the manner of loading of the prior art rotary retort furnaces often defeats the advantageous conveying mechanism of the rotary retort in that because of improper and uneven loading, jams of parts are formed at the charge end which cannot be broken up by the continuous tumbling in the rotary retort.
It has long been the practice in the prior art to support a retort for rotation within a heated shell at both ends of the retort or along the entire length of the retort. Such support in the heat treating furnace environment has significant economic disadvantages not only with respect to the original manufacture of it but also in connection with its maintenance. If the retort is supported at both ends outside of the furnace shell to facilitate maintenance then two heat and atmosphere seals will be required. Aside from the initial expense of such duplicate seals, in order to keep the seals effective they must be frequently repaired or replaced. If the support is within the heated shell, it presents serious maintenance problems.
Since rotary retort furnaces are often used in a controlled atmosphere heat treating operation, it is important to minimize any loss of the atmosphere within the furnace. In addition to losses of atmosphere and heat through the seals between the shell and the rotary retort, significant amounts of the atmosphere are lost in prior art furnaces during the charging of the retort.
Manufacture of the internal helical auger flight for a rotary retort furnace can prove to be an expensive aspect of the cost of construction of such a furnace. Inasmuch as rotary retort furnaces are usually used for handling small parts such as screws and nuts, the outer edges of the auger flight must be kept in close contact with the walls of the retort, otherwise parts will tend to become lodged between the auger flight and the wall. If parts heat treated in one operation become lodged in the spaces between an auger flight and the walls of a retort and subsequently drop into a different set of parts being heat treated in a subsequent operation, it can prove to be a burdensome and expensive task for the heat treater to have to separate such parts. Prior art means of forming an auger contiguous with the walls of the retort, such as machining, casting, or continuous welding of the auger flight edge, are very expensive.